The subject of the present invention is a multipath navigation signal processing method adapted to a receiver having a plurality of receiving antennas.
In a navigation receiver of the direct-sequence-code-division multiple-access and pseudo-range estimation type, the reflections which introduce a delay of less than one bit of a pseudorandom noise sequence (or xe2x80x9cchipxe2x80x9d) are liable to introduce a large time error. In the case envisioned here, a receiver having a plurality of receiving antennas, the attenuation of the effects of these multiple paths depends on the number of antennas in the receiver since each antenna generates a linear combination between the direct signal and the signals corresponding to multiple paths, and this information can be used by an attenuation algorithm.
Conventionally, the term xe2x80x9cchannelxe2x80x9d will refer to the signal coming from one antenna and a receiver will be called single-channel or multichannel depending on whether it has one or more antennas.
From a theoretical standpoint, the principle of maximum probability is used to estimate the time, and in particular the DDLLs (Digital Delay Lock Loops) constitute an approximation of this principle for a signal and a channel in white noise. The use of the principle of maximum probability results, in the case of the aforementioned reflections, in a nonlinear equation and the signal processing is correspondingly complicated thereby. MEDLLs (Multipath Estimating Delay Lock Loops) are a good example of this: they require at least twice as many correlators as the aforementioned DDLLs and to solve the nonlinear equations requires an appreciably longer time than the advance/delay correlation of a DDLL.
The MEDLL technique is described in particular in the article by Brian Townsend et al., entitled xe2x80x9cL1 Carrier Phase Multipath Error Reduction Using MEDLL Technologyxe2x80x9d, published in 1995 in xe2x80x9cProceedings of the Institute of Navigation GPSxe2x80x9d, pages 1539-1533, as well as in the article by Richard Van Nee et al., entitled xe2x80x9cThe Multipath Estimating Delay Lock Loop: Approaching Theoretical Accuracy Limitsxe2x80x9d, published in 1994 in the journal IEEE, pages 246-251.
A technique for attenuating the multipath effects using the MEDLL technique is also described in the article by G. El Sayed et al., entitled xe2x80x9cDesign of GPS Receiver Code and Carrier Tracking Loops for Multipath Mitigationxe2x80x9d, published in 1998 in Proceedings of the 11th International Technical Meeting of the Institute of Navigation, Nashville, pages 1041 to 1052. This technique is applied only to a single signal and is therefore suitable only for a single-channel receiver.
In general, a DDLL loop correlates the input signal with a locally generated replica. The loop then chooses, among all the possible correlations, as one for each delay, that which is closest to the input signal. The distance between two correlations is conventionally measured using a vector, which results in the solution of a linear minimization problem.
The present invention is provided for achieving a similar formulation in the case of a multichannel receiver.
The invention thus relates to a method of processing navigation signals for a satellite navigation system using a receiver, characterized in that the receiver has a plurality of antennas generating signals representative of the reception of signals corresponding to direct paths and to reflected paths and in that it carries out a search for the delays of the signals corresponding to direct paths and to reflected paths by comparing them with reference signals corresponding to known delays.
One particularly advantageous embodiment of the invention relies on the notion that the input signals of the various channels cover the same vector subspace as the direct signals and the signals coming from reflections.
It is therefore possible to estimate this subspace and then to use the distance between the subspace and the local correlations in order to formulate the minimization problem.
The method is therefore characterized in that it comprises:
a) a preprocessing of each antenna signal so as to filter it, by a filter matched to the waveform of the navigation signals of said navigation system and to its spectrum spreading code, and to take an average over the peaks of the filtered signals;
b) a decomposition into singular values in order to make an estimation of a vector signal subspace S from the filtered and averaged signals representative of the correlation peaks;
c) a one-dimensional search for the delays of the signals corresponding to direct paths and to reflected paths, by seeking the delays corresponding to minima in the distance between locally generated mode vectors corresponding to reference signals, the delays of which are known, and the vectors of the subspace S.
Said decomposition into singular values is preferably carried out for samples located around a correlation peak and in which most of the energy of the signal is concentrated. It may employ detection of a discontinuity in the singular values in order to estimate the rank p of the subspace S of the signal.
The method may be characterized in that said one-dimensional search is carried out in an estimated delay timeslot in which signals corresponding to a direct path and signals corresponding to reflected paths can be found.
Said one-dimensional search may comprise:
c1) a calculation in said delay timeslot of the inverse of the distance between the vectors of the subspace S and the locally generated mode vectors;
c2) an estimation of the delays of the signals corresponding to a direct path and of the signals corresponding to reflected paths by choosing the delays corresponding to the mode vectors for which the values of said inverses of the distance are a maximum.